AGENDA ITEM NO: 5
BOSTON BOROUGH COUNCIL
REPORT TO CABINET
15th December 2010
SUBJECT:
BOSTON BOROUGH STRATEGIC FLOOD RISK
ASSESSMENT 2010 (SFRA)
PORTFOLIO HOLDER:
COUNCILLOR RICHARD DUNGWORTH,
REGENERATION, PLANNING, SPORT AND
CULTURAL SERVICES
REPORT BY:
HEAD OF PLANNING AND STRATEGY
Reason For Report:
To note the completion and content of an updated report which will form a key element of the Local
Development Framework evidence base, and tool to aid future development management, and to adopt
as an evidence base on that basis. A copy of the report has been placed in the Members room and the
main diagrams will be displayed at the meeting.
Summary:
Revisions to Planning Policy Statement 25 – Development and Flood Risk (PPS25) highlighted that the
Borough’s 2002 SFRA would require updating to reflect more advanced forecasts in relation to the
potential effects of climate change on flood risk.
The 2010 SFRA therefore provides additional analysis in relation to consequence in the case of an
assumed worst case failure in the flood defence at 2115, taking forecast climate change impacts over
the next hundred years into account.
The Assessment also contains consideration of the relative probability of flooding at the present time,
from the sea, fluvial and other sources, taking into account current conditions and the state of flood
defences and drainage infrastructure found in the Borough. The area is protected by well maintained
defences of a good standard and the adopted Shoreline Management Plan policy is to hold the line of
the current defences. The planned Boston Barrier will in the future add to the standard of protection to
the town and adjoining areas.
The findings are an essential aid to development management decisions – the grant or otherwise of
planning permission for development – and aid the preparation of Local Development Plan Documents.
It will also assist promoters of development and developers in their submissions to the Council.
The updated work explains the degree of anticipated flood hazard at year 2115 in relation to an
assumed 1 in 200 year flood event and the extent and distribution of such assumed hazard across the
Borough in terms of zoning. Hazard classification in this context is a function of potential flood depth,
flood velocity and the possible likelihood of debris that is assumed could be present during a flood
event, and therefore varies with velocity and depth. Low hazard therefore denotes caution but with low
risk to people. Danger for some denotes risk to the vulnerable such as children, the elderly and infirm;
danger for most denotes risk to the public in general; and danger for all is classified as extreme hazard
including for the emergency services.
The analysis of the relative probability of land flooding from tidal or fluvial (river) sources in any one
year has been based on the results of hydraulic modelling with flood defences in place. Therefore low
probability denotes less than 1 in 1000 or more years (less than 0.1% chance); medium probability
denotes for tidal from 1 in 200 to 1 in 1000 years (0.1% to 0.5% chance), for fluvial from 1 in 100 to 1 in
1000 years (0.1% to 1% chance); and high probability denotes for tidal 1 in 200 or less years (greater
than 0.5% chance), for fluvial 1 in 100 or less years ( greater than 1% chance).
In summary the assessment conclusions therefore include that :
o Bicker and Swineshead have the least flood risk issues including a majority of land at low flood
hazard;
o Wrangle and Sutterton have land with a low probability of flooding but land in the danger for some
category;
o Kirton has land with a low probability of flooding but land in the danger for most category;
o Fishtoft, Freiston and Old Leake have land with a low probability of flooding but land in the danger
for all and low hazard categories;
o Butterwick has land with a low probability of flooding but land in the danger for all and danger for
most categories;
o Boston town has land that ranges from high to low probability of flooding and land in all flood
hazard categories – danger for all, danger for most, danger for some, low hazard.
Recommendation(s):
That Cabinet note and adopt the revised content of the updated SFRA, as part of the Local
Development Framework evidence base and tool to support development management decisions, and
aid to potential applicants and promoters of development in formulating proposals.
Reasons for Recommendations:
There are no specific planning policy changes or recommendations arising from the completion of the
technical analysis of the SFRA at this time.
Alternative Options:
In areas subject to the risk of potential flooding an up to date SFRA is required to ensure compliance
with national planning policy objectives when deciding planning application proposals. The Council
could be subject to risk of challenge if it continued to rely wholly on the 2002 SFRA for planning work.
Background Papers:
Boston Borough Council Strategic Flood Risk Assessment 2002.
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Implications:
Relationship to Community Plan
The SFRA helps inform the preparation of the LDF the means by which the spatial (land use
and development) elements of the Council’s Community Plan are delivered. This will include
the identification and allocation of sites for development needed to ensure the long term
sustainability and well being of the local community.
Financial
Budgetry provision has been made for staged payments for this key evidence base
document. The agreed overall cost for the work is £39,255 of which the Environment Agency
contributed £5,000.
Impact on Performance
The SFRA is an important tool in assisting sound development management decisions and
will aid the expeditious preparation of the LDF to the benefit of achieving the strategic
aspirations and objectives of the Council.
Human Resources
None.
Equality Impact Assessment
None as it is a factual and technical analysis related to flood risk in the Borough.
Risk Management
Flood risk issues have been identified as one of the key strategic risks of the Council as part
of the Corporate Plan for 2010/2011. The SFRA will aid in the understanding of such risks by
providing updated analysis. Reference to updated analysis will also reduce the risk of
unsound development management decisions and the possibility of subsequent challenges
to those decisions.
If the preparation of the LDF is not underpinned by an up to date and relevant evidence base
the strategy and policies put forward in development plan documents may be found to be
‘not sound’ after a public examination.
Legal
PPS25 specifies that a Local Planning Authority should carry out a SFRA to inform the
preparation of Local Development Documents which are part of the LDF. The Planning Acts
specify that the Local Planning Authority must prepare a local development plan for its area,
namely the LDF.
Environmental
The SFRA is a key piece of environmental evidence base that will, with other evidence,
ensure that the LDF takes into account and seeks to respond to environmental issues in a
beneficial way.

Wards
The SFRA covers the whole Borough and therefore provides information of relevance to all
wards.
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Main Report:
1.
Introduction
1.1
In Local Authority areas that are subject to the risk of potential flooding Strategic Flood Risk
Assessments (SFRAs) are prepared for two purposes:
 To help in development management decisions – the grant or otherwise of planning
permission for development; and
 To aid the preparation of Local Development Plan Documents.
1.2
The SFRA uses up to date data and evidence to help understanding of the likely consequences
stemming from flooding and the probability of such events occurring.
2.
Background
2.1
Since the 2002 Boston SFRA was prepared, changes to the national planning policy approach to
flood risk set out in revised Planning Policy Statement 25 – Development and Flood Risk
(PPS25), mean the scope of the findings in the 2002 report would no longer be up to date, and
therefore unlikely to be fully compliant with the evidential requirements set in the revised policy
framework.
2.2
Compliance is a significant consideration in ensuring the Council as local planning authority can
rely on sound evidence in the circumstances of planning appeals where decisions may turn on
the issue of flood risk. Equally important the SFRA is a key element of the evidence base that the
Council and others will rely on in formulating and evaluating future development options, and
ideas for including sites in the Local Development Documents (LDDs). Reference to the use of
an up to date SFRA in the evidence base is one key component in demonstrating that the
chosen development strategy is founded on credible evidence and can be relied upon to be
‘sound’ based upon a good understanding of conditions in the Borough. As a freestanding
assessment the SFRA will also contribute to the Sustainability Appraisal of the Councils’ plans as
laid down in the Planning and Compulsory Purchase Act 2004.
3.
The 2010 Assessment
3.1
The revised assessment builds on the findings of the 2002 report by updating work designed to
identify the potential circumstances of flooding at year 2115, taking into account climate change
assumptions. The year 2115 is chosen on the basis that modern built development would have
an expected life of around 100 years. The assessment is prepared in accordance with the advice
in the PPS25 Practice Guide, the sister publication to the main PPS25 document.
3.2
In a defended floodplain area, such as at Boston, there is always an assumed residual risk that a
failure in the flood defence could take place. The principle of ‘precautionary approach’ which
underpins the national policy approach to matters of flood risk requires that the worst case
consequences of such an event are assumed, assessed and taken into account. The updated
SFRA incorporates these findings as a series of flood hazard assumptions.
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3.3
Another component of the assessment is an examination of the relative probability of flooding
based on the current conditions, drainage infrastructure, and state of flood defence provision
found in the Borough. The condition of the flood defence is good and well maintained and the
policy in the current Shoreline Management Plan is to hold the defence line. Planned defence
works such as the Boston Haven Barrier will add to the standard of defence for the town and
adjoining areas when completed. The probability of flooding from tidal, fluvial, and other sources
is considered.
3.4
The assessment examines the above considerations as they affect the Borough as a whole, and
in relation to specific areas focused on and adjoining the larger settlements.
3.5
There are no development policy recommendations specific to the completion of the updated
SFRA, which will function as a stand alone reference/evidence tool in the same fashion as the
2002 assessment.
Summary of Findings/Conclusions
3.6
The main potential causes of flooding are described in detail as including overflowing of
watercourses, breaching of embankments, mechanical, structural or operational failure,
floodlocking and tidelocking and localised flooding. The potential sources of flooding within
Boston Borough which are considered to present a strategic flood risk are: Wash Banks
 Boston Haven (& Witham Outfall Channel)
 Welland Estuary (Outfall Channel)
 River Witham (upstream of Grand Sluice)
 South Forty Foot Drain
 North Forty Foot Drain
 Old & New Hammond Becks
 Maud Foster Drain
 Hobhole, Cowbridge and Frith Bank Drains.
Each of these was described in detail in the 2002 SFRA and their significance as a primary or
secondary source evaluated. Those considered to be a primary flood risk were the River Witham,
South Forty Foot Drain, Boston Haven and the Wash Banks.
3.7
Other sources of flood risk are identified as Internal Drainage Board Arterial Drains and other
pump drained systems along with a number of potential minor sources. These include
groundwater flooding associated with a rising water table the result of long-term high rainfall
totals; surface water flooding when excess rainwater runs off across the surface of the land; and
sewer flooding which is generally restricted to urban areas.
3.8
A number of flood risk management measures in place reduce the level of flood risk in the
Borough. These include flood storage areas at places along the River Witham, and raised
defences – earth embankments or concrete floodwalls. Along the River Witham in Boston
Borough the condition of the raised flood defences are rated as very good (75%) or good (25%).
Along the South Forty Foot Drain the ratings are very good (35%), good (53%), or fair (12%).
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3.9
The Environment Agency flood zone classification place the majority of the Borough area within
Flood Zone 3. Therefore in order to be able to apply the Sequential Test and Exception Test as
required in PPS25, in such areas, two sets of information/analysis are presented:
Flood Hazard Zones –
the consequences should a breach occur in the raised
defences taking climate change assumptions into account;
Relative Probability of Flooding – the likelihood of a flood event given the presence of the
defences.
3.10 The risk based Sequential Test, explained in PPS25, is applied at all stages in planning and the
defined flood zones are the starting point. Simply put, development sites should be chosen
preferentially in Flood Zone 1, then Flood Zone 2, and only where there are no sites available in
the former two should the suitability of sites in Flood Zone 3 be considered, applying the
Exception Test if required. The PPS25 Flood Zones are based on the probability of river and sea
flooding to which an area of land is currently subject, ignoring the presence and effect of existing
flood defences or other man-made obstructions to flood flows. Flood Zone 3 has the highest
annual probability of flooding and Flood Zone 1 the lowest.
3.11 The Exception Test should only be applied after the Sequential Test to evaluate sites for
development and to draft policies against which to consider planning applications. For the
Exception Test to be passed:1. It must be demonstrated that the development provides wider sustainability benefits to the
community that outweigh flood risk, informed by a SFRA where one has been prepared.
2. The development should be on developable, previously developed land or, if it is not on
previously developed land, that there are no reasonable alternative sites on developable
previously developed land
3. A Flood Risk Assessment must demonstrate that the development will be safe, without
increasing flood risk elsewhere, and, where possible, will reduce flood risk overall.
3.12 Technical guidance referred to in the PPS25 Practice Guide identifies four flood hazard zones,
Low Hazard / Danger for Some / Danger for Most / Danger for All. These definitions are therefore
used for the purposes of the SFRA. The relative probability of flooding has been based upon the
results of hydraulic modelling of fluvial and tidal flood events. The results show the relative
probability of land flooding with the defences in place, represented as low probability, medium
probability, or high probability.
3.13 The hazard zone is simply the area behind a flood defence line within which people could be at
risk following a breach in that defence line. Since a breach is assumed to occur either as a result
of overtopping or as a result of a premature (residual risk) failure of a flood defence, the hazard
zone is not associated with a specific probability of occurrence. It is a worst case representation
of the hazard zone envelope derived from the assumption of a series of breaches at intervals
along the primary defence line in that area.
3.14 The available topographical information used in the breach scenario models to establish the
hazard zone envelopes includes data derived from airborne laser imaging. This coverage was not
as comprehensively available at the time of the 2002 SFRA. Ordnance Survey data including
radar coverage obtained from aircraft or satellites is also available for the hazard mapping.
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3.15 The relative probability of flooding has been taken from the results of the hydraulic modelling of 1
in 100-year (1% chance in any year) and 1 in 1000-year (0.1% chance in any year) fluvial flood
events in the principal fluvial flood risk sources and hydraulic modelling of the 1 in 200-year
(0.5% chance in any year) and 1 in 1000-year (0.1% chance in any year) tidal flood events in the
Wash and Boston Haven.
3.16 Where normally development might be expected to occur in the Borough over the next 5 to 10
years, such as the main settlements, a more detailed analysis of the flooding issues is presented.
The assessment presents the main analysis on two sets of maps. The sets show flood hazard
zones should a worst case breach occur in the raised defences taking assumed climate change
into account at year 2115, and the relative probability of flooding at the present given the
presence of the defences. The degree of flood risk throughout each of the 10 study areas is
assessed from a combination of factors, sources of information and engineering judgement.
3.17 Flood risk sources considered all open watercourses and, where applicable, principal surface
water and combined (foul + surface water) sewers. Possible flooding from foul sewers is not
assessed as this can occur from a variety of causes, often with no direct or quantifiable
relationship to extreme rainfall events. Flooding from groundwater and canals is considered of no
relevance in the Boston area.
3.18 The potential increased flood risk posed by the urbanisation of a ‘greenfield’ development site to
others areas downstream of the development site also has to be evaluated. This risk can arise
not only from the additional runoff volumes and higher peak runoff rates generated by newly
impermeable areas but also from the reduction in natural floodplain storage capacity if the
development takes place in a floodplain.
3.19 The individual assessments for the 10 study areas are dealt with under the following headings:1.
2.
3.
4.
5.
General description of the study area
Hydrology of the study area (including hydraulic structures, etc.)
Flood hazard zones (2115), within the study area
Relative probability of flooding (current situation), within the study area
Flood risks to downstream areas.
3.20 The flood risk assessments of the study areas made for strategic planning purposes do not
preclude the necessity for site-specific flood risk assessments of individual development sites
within the wider study areas. The assessments of the study areas will nevertheless be used as a
general framework within which site-specific flood risk assessments are undertaken.
3.21 The individual study area findings can be summarised as:
Boston Town Study Area – almost entirely in flood zone 3, with a variation in the flood hazard
from danger for most to danger for all, and areas where the probability of flooding is lower than
others.
Bicker Study Area – entirely in flood zone 1, outside flood hazard zones, and with a low
probability of flooding.
Butterwick Study Area – entirely in flood zone 3, with a variation in the flood hazard between the
predominant danger for most to danger for all areas, but has a low probability of flooding.
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Fishtoft Study Area – entirely in flood zone 3, with a variation in the flood hazard between the
predominant danger for most to danger for all areas, but has a low probability of flooding.
Freiston Study Area – entirely in flood zone 3, flood hazard is danger for all with a small area of
low hazard, but has a low probability of flooding.
Kirton Study Area – entirely in flood zone 3, flood hazard is danger for all with a small area of low
hazard, but has a low probability of flooding.
Old Leake Study Area – entirely in flood zone 3, flood hazard is mainly danger for all with the
remainder danger for most, but has a low probability of flooding.
Sutterton Study Area– entirely in flood zone 3, flood hazard varies with some areas in danger for
some and other areas outside of the hazard zones, but has a low probability of flooding.
Swineshead Study Area – mainly in flood zone 1, mainly low flood hazard and has a low
probability of flooding.
Wrangle Study Area – entirely in flood zone 3, mainly low flood hazard with pockets of land in the
danger for some category, but has a low probability of flooding.
3.22 The overall assessment also provides guidance on how to manage flood risk through the design
of development, and the use of Sustainable Drainage Systems (SUDS).
3.23 A full copy of the Assessment report has been placed in the Members Room, and
Diagrams/Plans illustrating the main features of the findings will be on display at the meeting.
Should individual members require a full copy of the report in time for the meeting early
notification would be essential in order to ensure printing time for the numerous colour elements
of the document can be timetabled.
4.
Recommendation
The 2010 Assessment of flood risk is an essential part of the evidence base for creating the LDF.
Members are therefore asked to note and adopt the technical analysis contained within it as part of the
evidence base upon which the formulation of planning policies in the LDF will partly rely, and as an aid
to development management decisions.
Review Date:
SFRA may require future updating as conditions change but no review date is suggested as relevant at
this time.
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